1. Technical Field of the Invention
This invention relates to optical networks and in particular to systems and methods for scheduling of transmission by optical nodes in a passive optical network.
2. Description of Related Art
Access networks provide connectivity from a network user to an edge of a network. Different types of access networks include digital subscriber line, hybrid fiber coax (HFC) and more recently fiber to the home. Fiber to the home generally employs a passive optical network (PON) with optical network units (ONUs) installed at network users' premises coupled to an optical line terminal (OLT) at a central office or network provider.
Passive Optical Networks include various protocols, including ATM-PON (APON), Broadband PON (BPON), Ethernet-PON (EPON) and Gigabit-PON (GPON). APON is based on an ATM protocol while BPON protocol is based on APON with added support for WDM and other features. EPON is described in IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CMSA/CD) Access Method and Physical Layer Specifications. EPON utilizes internet protocol (IP) having variable-length Ethernet frames. GPON is described in ITU-T G.984 Gigabit-capable passive optical networks (GPON) standard and supports ATM, IP and Time Division Multiplexed (TDM) protocols (such as SONET and SDH) with a choice of ATM, Ethernet and GPON encapsulation method (GEM) frames. With demand for greater bandwidth by network users, PON protocols are migrating to support increased data speeds. EPON protocols require 1 Gb/s data rate while current GPON standards require a rate up to 2.5 Gb/s. In addition, 10 Gigabit Ethernet PON (10G-EPON) is an emerging standard in IEEE 802.3av requiring an operating rate of 10 Gb/s. Additional PON technologies, such as WDM-PON, are also being developed.
Due to the topology of a PON, the transmission modes for downstream (i.e., from OLT to ONU) and upstream (i.e., from ONU to OLT) are different. In the downstream direction, the OLT broadcasts an optical signal in a 1×N nature to ONUs in a continuous mode (CM). The ONUs extract data frames which are addressed to their destinations from the optical signal. In the upstream direction, the optical signals transmitted by the ONUs are converged into one fiber or channel by an optical coupler such that the ONUs must share upstream channel capacity. To avoid collision and promote bandwidth sharing, ONUs asynchronously transmit in optical bursts during allocated time slots. Since the distances between the OLT and ONUs are not uniform, the phases of the optical bursts received by the OLT differ from packet to packet, and the optical bursts received by the OLT have different power levels. In some applications, a range of approximately 20 decibels (dB) have been observed. For example, in ITU-T recommendation G.984.2 a 15 dB differential optical path loss is defined. In the same standard the maximum variation of the mean launched power of the upstream transmitter is defined as 5 dB. These two numbers together lead to a maximum power difference of 20 dB at the upstream receiver. This difference in power levels creates problems for optical receivers in the OLT.
As such, there is a need to compensate for the variation between received optical bursts in an optical line terminal of a passive optical network.